Design Idea DI-59
LinkSwitch® Low Cost 2.5 W
CV/CC Charger or Adapter
Topology
Device
Power Output
Input Voltage
Output Voltage
Application
Flyback
Charger/Adapter
2.5 W
85-265 VAC
LNK500
5.5 V
T1 drives current out of the secondary winding, forward
biasing D6, charging C6 and developing/maintaining the
output voltage across C6.
Design Highlights
• Replaces a linear transformer based power supply at the
same or lower cost, with better output regulation
The RCD network of C3, D5 and R1 has two functions:
1) It clamps the reflection of the output voltage (VOR) on the
primary winding, as the MOSFET turns OFF.
2) It holds VOR, for use as output feedback, eliminating the
cost and complexity of a separate feedback path.
• No-load input power consumption is less than 300 mW, at
230 VAC input: meets worldwide conservation guidelines
• Extremely simple circuit – requires only 23 components!
• No alternate path of secondary-side feedback is required:
output feedback is obtained through the main transformer!
• Output Voltage (CV) tolerance ±10% at peak power point
• Output Current (CC) tolerance ±25%, when L ≤ ±10%
• Featuresthefollowingauto-recoveringprotectionfunctions:
output short-circuit, open feedback loop, thermal shutdown
• Typically about 68% efficient!
Resistor R1 attenuates the switching noise from the VOR.
Resistor R2 determines the amount of feedback current that
flows into the CONTROL pin of U1. The output voltage can
be fine-tuned by varying the value of R2. CONTROL pin
capacitor C4 stores power and supplies it back to U1 during
start-up, determines the restart “attempt” frequency in the
auto-restart mode, shunts high frequency noise around U1
and provide U1 with the instantaneous MOSFET gate-drive
current it requires. Resistor R3, capacitor C5, the 22-turn
core cancellation winding and the 5-turn shield winding all
reduce EMI.
• Meets EN550022 B EMI limits, without a Y-1 type Safety
capacitor between the primary and the secondary
• Ultra-low leakage current: < 5 µA at 265 VAC input
Operation
Fusible resistor RF1 gives short-circuit fault protection and
limits start-up inrush current. Diodes D1–D4 provide full
bridgerectificationthatchargescapacitorsC1andC2. Inductors
L1andL2andcapacitorsC1andC2formalow-costpi(π)filter
that attenuates conducted EMI. Transformer (T1) winding
phasing and D6 orientation let no secondary winding current
flow when the MOSFET in U1 is ON, so the current that flows
through the primary winding stores its energy in the core of T1.
When the MOSFET in U1 is turned OFF, the energy stored in
LinkSwitch based solutions are designed to operate only in
2
~
the discontinuous conduction mode. PO 0.5 L I f, where
~
PO = Output Power, L = transformer primary inductance,
I = LinkSwitch peak current, f = Switching frequency and
η=Efficiency. I2fisaccuratelycontrolledfortheLinkSwitch;
therefore, PO is proportional to L.
T1 EE13
L1
1.5 mH
LP = 2.3 mH
LinkSwitch
5.5 V, 0.45 A
D
S
1
5
C4
90T, #36
22T, #34
15T, #30, TIW
C
U1
0.22 µF
25 V
LNK500
2
6
2
C2
4.7 µF
400 V
C6
330 µF
16 V
D1-D2
1N4007
C3
100 nF
63 V
R2
17.4 kΩ
1%
5T,
#30 x 3
R5
L
D6
22 kΩ
FILM
3
SB1100
85-265
VAC
C1
R3
N
4.7 µF
400 V
D5
RTN
RF1
100 Ω
1N4937
8.2 Ω
1 W Fusible
C7
R4
C5
100 pF
500 V
R1
100 Ω
150 pF
100 V
33 Ω
L2
Ferrite
Bead
D3-D4
1N4007
PI-3692-091903
Figure 1. 2.5 W LinkSwitch Based Charger/Adapter
September 2003
DI-59
www.powerint.com